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Supersonic Sounds plc Internal Correspondence
Not to be quoted in published work
May 2000
Supersonic Sounds plc Design Department Technical Report
Design ideas for the “Anti-Heckler”
microphone pre-amplifier
Michael Prior-Jones
Summary
This report details work done to date on the “Anti-Heckler”- a microphone pre-amplifier
intended to be used by people speaking in public. It features a sine wave test tone
generator and peak-detecting output level meter. The circuitry has been designed to
minimise the use of operational amplifiers through the use of bipolar transistor
techniques. This has been done to avoid a supply difficulty with the TL071 opamp, but if
this passes, I would recommend the replacement of the transistor circuitry (particularly
the differential input stage) with the higher-quality opamp alternative. Much work has
still to be done to test the system and check that the individual subsystems do not
interfere with each other.
Page 1 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
Supersonic Sounds plc Internal Correspondence
Not to be quoted in published work
May 2000
Supersonic Sounds plc Design Department Technical Report
Design ideas for the “Anti-Heckler”
microphone pre-amplifier
Michael Prior-Jones
Contents
1
Introduction ...........................................................................................................................3
1.1
Background ...................................................................................................................3
1.2
Requirements ................................................................................................................3
1.3
Product Specification ..................................................................................................4
1.4
Development Plan .......................................................................................................4
1.5
Report Structure ...........................................................................................................4
2
Design considerations ..........................................................................................................5
2.1
Microphone input stage ..............................................................................................5
2.2
Line output stage..........................................................................................................7
2.3
Sinewave oscillator .......................................................................................................8
2.4
Output level meter .......................................................................................................9
3
Conclusion .......................................................................................................................... 10
3.1
Recommendations .................................................................................................... 10
3.2
Final Circuit Design .................................................................................................. 11
4
References ........................................................................................................................... 12
Page 2 of 12
Supersonic Sounds plc Internal Correspondence
Not to be quoted in published work
May 2000
Supersonic Sounds plc Design Department Technical Report
Design ideas for the “Anti-Heckler”
microphone pre-amplifier
Michael Prior-Jones
1
Introduction
The “Anti-Heckler” is a new proposal for a microphone pre-amplifier, giving a line level
output intended for use with power amplifiers like the “Discotomax”. It also
incorporates a number of other features designed to assist the user in setting up a public
address system.
1.1
Background
This product grew out of a proposal from the Sales Department to make a microphone
pre-amplifier intended for use for public speaking and lecturing. This would be partnered
with an appropriate power amplifier and loudspeakers to make a compact public address
system. A number of requirements were identified by Sales through consultation with our
customers, and these are detailed below.
1.2
Requirements
The requirements identified were:
 amplification of a conventional 600 balanced-line microphone to line level.
 provision of a sine wave test tone to allow the system be set up single-handedly.
 provision of an output level meter to give a visual indication of sound level.
Page 3 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
1.3
Product Specification
During discussion with the Sales Director, more detailed specifications for the design
were agreed upon and these are detailed below:
 Microphone input at 1mV peak-to-peak with 600 input impedance.
 Differential amplifier to handle balanced-line microphones and reduce noise
when working with long microphone cables.
 Internal sine wave oscillator at 400Hz ± 7.5%
 Provision to switch between the microphone and the sine wave oscillator.
 Low impedance output to power amplifier at 775mV r.m.s.
 Output signal level meter.
Due to supply problems with the TL071 operational amplifier chip, the decision was
taken to minimise the use of op-amps in the design, and use discrete transistor circuitry
instead.
1.4
Development Plan
This report details the initial considerations and circuit designs, backed up with
laboratory experiments. Further work has to be carried out in prototyping and
modifications may be necessary to the circuit designs as they appear here.
1.5
Report Structure
Section 2 describes the design process for each of the circuits within the overall system.
Section 3 gives the full circuit diagrams and raises issues that need to be further
investigated before the product can go into production.
Page 4 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
2
Design considerations
The design process is made considerably more complicated by the requirement to use
bipolar transistors rather than op-amps wherever possible. The bipolar transistor is
considerably less ideal than the op-amp, and consequently considerably more
mathematics has to be used in the circuit designs.
The circuit is divided into a number of subsystems, which are shown in the block
diagram of figure 1.
Microphone
input stage
Line output
stage
Switch
Sinewave
oscillator
Level Meter
Figure 1- Outline block diagram of system
2.1
Microphone input stage
The microphone input stage is a differential amplifier, to reduce common-mode noise
introduced by the long microphone cable. A long-tailed pair is used to implement this
stage using a matched pair of bipolar transistors. These may be discrete components,
matched by hand (a laborious process) or an IC. The circuit has a gain (determined
experimentally) of approximately 43dB (130 times) over a bandwidth of 100Hz-10kHz,
which is acceptable for speech. Our 1mV pk-pk signal is thus amplified to 130mV pk-pk,
which is 90mV r.m.s. The output level is 775mV r.m.s., and thus we need a further gain
stage of 8.6 (19dB) to reach the correct level.
The circuit is adapted from Tew, Howard & Garner1 (page 51) and has a common-mode
rejection ratio (CMRR) of 49dB, which is likely to be adequate for this application. Large
values of CMRR are desirable to remove noise picked up on the microphone leads. The
CMRR may be increased by substituting a current source for RT, but my opinion is that
this is unlikely to be necessary.
The signal from the microphone will arrive on screened twisted-pair cable, terminated in
an XLR connector. Pin 1 of the XLR should be connected to amplifier ground, and pins
2 and 3 connected to the differential inputs. The 620 resistor provides a close
impedance match with the cable and microphone (which are 600), to maximise sound
quality.
The DC blocking capacitors C1 and C2 are effectively in series, and form a high pass filter
with RIN. The values given give a –3dB point at 50Hz, giving us plenty of bandwidth.
The output impedance of the differential amplifier is equal to RC i.e. 6k8, and thus a
buffer stage should be introduced to avoid loading problems. This is implemented using
the emitter follower circuit shown in figure 3.
Page 5 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
Vcc (15v)
RC 6k8
C1 10F
Vout
(output)
TR1, TR2
BC182
Noninverting
input
RB1 10k
RB2 10k
RIN
620
0v
Inverting
input
C2 10F
RT 6k8
Vee (-15v)
Figure 2- The differential input stage
Vcc (15v)
RB1 100k
Vin (input)
Cin 1F
RB2
100k
TR3,
BC182
Vout
(output)
RE 6k8
0v
Figure 3- emitter follower buffer for differential amplifier
The emitter follower has approximately unity gain, but a very high input impedance and a
very low output impedance. It will adequately buffer the output of the differential
amplifier. The biasing has been arranged to be at the same level as the differential
amplifier, so no dc blocking capacitor is required between them.
A further gain stage is now required to bring the microphone signal up to line level. This
will be the common emitter amplifier of figure 4. It has an approximate gain of 9.1 times,
which will take the signal up to line level.
Page 6 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
Vcc (15v)
RB1 91k
Vin (input)
RC 7k5
Cout 10F
TR4,
BC182
Vout
(output)
Cin 1F
RB2 10k
RE 820
0v
Figure 4- the common emitter stage
The output resistance of this circuit is equal to RC i.e. 7k5. An additional stage of
buffering will be added to lower the output resistance before sending the signal out to
the power amplifier. The blocking capacitor on the output stops the master volume fader
from affecting the biasing.
2.2
Line output stage
Vcc (15v)
Vin (input)
RB1 100k
CIN 1F
TR3,
BC182
RFader
10k log.
RB2 100k
COUT 1F
Vout
(output)
RE 6k8
0v
Figure 5 - the emitter follower line output stage
This circuit (figure 5) forms a master volume control as well as reducing the output
impedance of the system to a low level (approx. 50) to drive the power amplifiers. The
blocking capacitors keeps the DC bias current from interfering with other parts of the
circuit.
Page 7 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
2.3
Sine wave oscillator
The 400Hz test tone is to be generated by a Wein bridge oscillator (from Angus, Tew &
Howard2, page 86) built using an opamp. The circuit shown in figure 6 oscillates at a
frequency of 408 Hz, which is well within our 7% limit (372-428 Hz), even allowing for
variations in resistors and capacitors. This circuit has not been exhaustively tested, and
some adjustments may be necessary to the values of R1, R2 and R3 to make the circuit
oscillate reliably. The output from this oscillator is a 10v peak sine wave, which must be
attenuated down to 775mV r.m.s. (a factor of 9, or –19dB gain). This is done by the
potential divider of R4 and R5.
R6
390k
C1
1nF
R7
390k
TL071
±15v
supply
+
-
C2
1nF
R1
3k3
R3
10k
Figure 6- The sine wave oscillator
Page 8 of 12
R2 1k
R4
82k
R5
10k
Vout
0v
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
2.4
Output level meter
An output level meter gives a visual indication of sound level, and can provide an early
warning of amplifier distortion. The classic “vU meter” is simply a calibrated voltmeter
across the output of the amplifier, and is difficult to use because the meter needle moves
very rapidly. This can make it very difficult to see the level in fast-paced music, or spot
short transient peaks. Peak detector meters (sometimes called Peak Programme Meters
or PPMs) were invented by the BBC to avoid this problem. Early models were electromechanical in nature, but improvements in technology have made it possible to build
similar systems electronically. Figure 7 shows a peak detector circuit taken from Angus,
Tew & Howard2 (p. 52). The circuit forms a half-wave rectifier, which charges the
capacitor. During negative half-cycles, the diode does not conduct, so the capacitor’s
only discharge path is through the 1M resistor. Consequently, the capacitor charges
rapidly during peaks and discharges slowly in between, making the display on the meter
much easier to read. The opamp is arranged to buffer the signal, and compensate for the
0.7v drop across the diode.
TL071
±15v
supply
Vin
+
R1
10k
C1
10nF
0v
Figure 7- the peak level meter
Page 9 of 12
R1
1M
V
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
3
Conclusion
The circuit designs presented here still need further work- some have been tested in the
laboratory, and others have not. The system will need testing as a whole to ensure that all
of the subsystems work together correctly and meet the specifications. The system also
needs a power supply (±15 volts DC) and this will need to be tested with the circuits
described above to ensure that mains hum and other undesirable interference does not
enter the sensitive audio circuitry.
3.1


Recommendations
The circuit should be packaged in a small desktop case, preferably with a sloping
top, incorporating the power supply and all connectors, and with the volume
fader and test tone switch mounted on top. All connectors and controls should
be clearly labelled.
If the supply situation on operational amplifiers improves, significant benefits
would be gained by replacing the transistor circuitry with the higher-quality
opamp alternatives. The differential amplifier in particular would benefit from the
use of an opamp to give higher common-mode rejection ratio and lower noise. I
would suggest the use of the LM833 or OP-37 low noise opamps.
Page 10 of 12
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
3.2
Final Circuit Design
Vcc (15v)
R5 6k8
R9 91k
C1 10F
TR1, TR2
BC182
Noninverting
input
R1
620
R12 7k5
R6 100k
R2 10k
TR3,
BC182
C3 1F
R3 10k
C510F
TR4,
BC182
To switch,
below
C4 1F
R1010k
R7 100k
R8 6k8
R11 820
0v
Inverting
input
C2 10F
R4 6k8
Vee (-15v)
R22
390k
C7
1nF
R23
390k
C8
1nF
R17
3k3
From C5
above
TL071
±15v
supply
Vcc(15v)
Switch
R13
10k log.
+
-
R19
10k
R14 100k
C6 1F
R18
1k
R20
82k
R21
10k
TL071
±15v
supply
R15 100k
TR5,
BC182
Vin
+
C9 1F
R16 6k8 Line out
0v
Page 11 of 12
R24
10k
C10
10nF
R25
1M
V
Design Ideas for the “Anti-Heckler” Microphone preamplifier.
4
References
1. Tew, Howard & Garner (1994) “First Year Laboratory Scripts, Spring Term
2000”, page 51
2. Angus, Tew & Howard (1992) “First Year Laboratory Scripts, Autumn Term
1999”, pages 86 and 52
Page 12 of 12